ptmx and pts - pseudo-terminal master and slave

The file /dev/ptmx is a character file with major number 5 and minor number 2, usually of mode 0666 and owner.group of root.root. It is used to create a pseudo-terminal master and slave pair.

When a process opens /dev/ptmx, it gets a file descriptor for a pseudo-terminal master (PTM), and a pseudo-terminal slave (PTS) device is created in the /dev/pts directory. Each file descriptor obtained by opening /dev/ptmx is an independent PTM with its own associated PTS, whose path can be found by passing the descriptor to ptsname(3).

Before opening the pseudo-terminal slave, you must pass the master's file descriptor to grantpt(3) and unlockpt(3).

Once both the pseudo-terminal master and slave are open, the slave provides processes with an interface that is identical to that of a real terminal.

Data written to the slave is presented on the master descriptor as input. Data written to the master is presented to the slave as input.

In practice, pseudo-terminals are used for implementing terminal emulators such as xterm(1), in which data read from the pseudo-terminal master is interpreted by the application in the same way a real terminal would interpret the data, and for implementing remote-login programs such as sshd(8), in which data read from the pseudo-terminal master is sent across the network to a client program that is connected to a terminal or terminal emulator.

Pseudo-terminals can also be used to send input to programs that normally refuse to read input from pipes (such as su(8), and passwd(8)).

R /dev/ptmx , /dev/pts/*

The Linux support for the above (known as Unix98 pty naming) is done using the devpts filesystem, that should be mounted on R /dev/pts .

Before this Unix98 scheme, master ptys were called R /dev/ptyp0 , ... and slave ptys R /dev/ttyp0 , ... and one needed lots of preallocated device nodes.

getpt(3), grantpt(3), ptsname(3), unlockpt(3), pty(7)